Perch set up in fact, so ordinary (5)
I believe the answer is:
roost
'perch' is the definition.
(I know that perch can be written as roost)
'set up in fact so ordinary' is the wordplay.
'set up' is a reversal indicator (in a down clue, letters go up).
'in' indicates a hidden word.
'tsoor' is hidden in the letters of 'fact so ordinary'.
'tsoor' backwards is 'ROOST'.
(Other definitions for roost that I've seen before include "Settle down (on a branch?)" , "where fliers land" , "settle for a nap" , "Place on which bird rests" , "Bird's night-time perch" .)